The present invention is directed to a method for detecting sources of ultra-trace metal contamination in the manufacture of silicon wafers and integrated circuits, in general, and a method for detecting such sources in individual process steps using a contamination monitor wafer, in particular.
As integrated circuit technology advances and the size of electrical devices continues to decrease, the need to impose tighter limits on spurious electrical current leakage continues to increase. These leakage paths are either through minority carrier recombination at metallically contaminated point defect sites in the vicinity of electrical junctions within the device or sub-device cell structure, or through leakage paths in the oxides used as field effect gates of isolation structures. Both paths are deleteriously increased in size when the silicon wafer is contaminated with metals such as iron, copper, nickel, chromium, zinc, and titanium, in general, and iron, in particular.
The manufacture of silicon wafers and electrical devices includes a number of individual processing steps which potentially contribute to metal contamination. Such steps include wet cleaning, ion implantation, reactive ion etching, gaseous cleaning, thermal treatment and polishing. Each of these individual processing steps should be examined, therefore, to determine whether or to what extent, it contributes to metal contamination.
Others have previously proposed that a contamination monitor wafer be used in combination with various minority carrier lifetime measurement or minority carrier diffusion length techniques as "tools" to detect sources of metal contamination. Minority carrier diffusion length and minority carrier lifetime are related according to the following expression: L.sub.d =((D.sub.n (.tau..sub.n)).sup.0.5 wherein L.sub.d is the minority carrier diffusion length in microns, D.sub.n is the election diffusion coefficient, taken to be 35 cm.sup.2 /sec, or 3500 .mu..sup.2 /.mu.s and .tau..sub.n is minority carrier lifetime in microseconds.
The minority carrier lifetime and minority carrier diffusion length of a silicon wafer are both related to the extent of metallic contamination of that wafer. Wherever references are made herein to minority carrier lifetime, it should be kept in mind that minority carrier diffusion length is for many purposes interchangeable therewith and may be substituted therefor. As disclosed in, for example, Zoth and Bergholz, J. Appl. Phys., 67, 6764 (1990), an increase in metal impurity content corresponds to a decrease in minority carrier diffusion length. According to this approach, therefore, the minority carrier lifetime, or minority carrier diffusion length of the monitor wafer is determined before and after it is subjected to one or more individual steps of a silicon wafer or electronic device manufacturing process. A comparison of the two lifetimes or lengths reveals whether, or to what extent, the processing step(s) contribute to metal contamination.
At present, there are several methods for measuring minority carrier lifetime in a silicon slice, slug or crystal, each involving the injection of excess carriers by some means. One technique involves the injection of excess carriers followed by the steady state measurement of an excess carrier dependent parameter such as surface photovoltage or photocurrent.
When making and interpreting the results of such measurements it is often assumed that metal contamination is the sole cause of minority carrier lifetime reductions and that the relative amounts of metal contamination introduced by a process step may be determined by the measurement of minority carrier lifetime. If, however, recombination paths parallel to and independent of metal contamination sites are introduced during the step which is being monitored, the interpretation of the data becomes difficult and the validity of such a minority carrier lifetime measurement to determine the amount of metal contamination introduced is greatly reduced or completely annulled. Oxygen precipitates are one such source of parallel recombination paths and they can dominate the recombination process if present in an amount greater than 10.sup.8 oxygen precipitates per cubic centimeter.
The typical oxygen content of Czochralski silicon also complicates the detection of metallic defects through the formation of thermal donor states which change the sample resistivity. As a result, Czochralski grown silicon is routinely subjected to a donor annihilation at an elevated temperature prior to measurement of the minority lifetime. Such elevated temperature (e.g., at least 650.degree. C.) heat treatments, however, are known to be strong contributors to metallic contamination--as small amounts of residual iron on the surface of the sample are driven into the sample, significantly reducing the lifetime. Donor annihilation is also disadvantageous because it can promote formation of the extended oxygen defects described above.